ElmoMC SimplIQ Software Manual - Manual (Page 119)

SimplIQ Software Manual Commutation
MAN-SIMSW (Ver. 1.4)
8-9
The selection rules for parameters I and f are as follows:
ƒ
The torque I must be as large as possible so as to reduce the relative effect of disturbance
torques (such as cogging and friction) on the resulting waveform. Normally, I is taken as
about 50% of the continuous motor current.
ƒ The frequency f must be selected so that the amplitude of the position sine is 6 to 8 bits.
The motor position can be analyzed accurately enough when the position sine amplitude
is 4 encoder bits or more. A slightly higher amplitude is set to prevent minor load
changes from disturbing the analysis. Larger position amplitudes will work, but the
shaft oscillation at the motor starting process will be unnecessarily large.
ƒ The frequency f must be such that in that frequency, the load behaves inertially. This
means that in that frequency, the phase angle φ(f) is in the range of [-140…-220] degrees,
and that the amplitude function A(f) does not have the high gradient of near resonance
regions. The algorithm will not function near a resonant frequency, or in a low
frequency in which a large viscous friction is present.
ƒ Specifying a very high oscillation frequency (CA[15] = -3 or -4) is not recommended,
because not enough position samples will be available for each sine cycle.
The Composer program normally selects the parameters I and f at the “Establishing
Commutation” stage of auto-tuning.
8.4.2 Method Limitation
The algorithm presented here is quite robust, and should work for most motor systems,
including those with moderate backlash. It does, however, have the following limitations:
ƒ
The encoder must have a resolution of at least 256 counts per pole pair. For example, if a
motor has three pole pairs, 256 lines (1000 counters/revolution) will suffice. An encoder
with 128 lines will not.
ƒ The system cannot be extremely unbalanced. It is essential that the motor does not
accelerate significantly when no current is applied.
ƒ The motor cannot be free to oscillate in both directions.
ƒ The motor static load should not change significantly. If the static load is subject to
considerable changes, tune the algorithm with the highest possible load. Namely, the
inertia of the load must be above 40% of its value from when the parameters were tuned,
and it cannot exceed 40% of its value from when the parameters were tuned. If the load
inertia is too high, the present torque level will not suffice for oscillating the motor shaft.
The algorithm will fail and the motor will not start.
ƒ The algorithm assumes that in the excitation frequency, the motor and the load behave
like inertia. This assumption fails in the following cases:
ƒ The excitation frequency is a resonant frequency of the system.
ƒ There is a large viscous (speed-dependent) friction and the excitation frequency is
so low that the phase of the transfer function between the torque and acceleration is
not in the range of [-30…30] degrees. If the transfer function phase is out of range,
the algorithm will fail and the motor will not start.